1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pump assembly journal bearings. More particularly, but not by way of limitation, one or more embodiments of the invention enable a press-fit bearing locking system, apparatus and method.
2. Description of the Related Art
Fluid, such as gas, oil or water, is often located in underground formations. In such situations, the fluid must be pumped to the surface so that it can be collected, separated, refined, distributed and/or sold. Centrifugal pumps are typically used in electric submersible pump (ESP) applications for lifting well fluid to the surface. Centrifugal pumps impart energy to a fluid by accelerating the fluid through a rotating impeller paired with a stationary diffuser. A rotating shaft runs through the central hub of the impeller and diffuser. A motor upstream of the pump causes the pump shaft to turn, and the impeller is keyed to the shaft such that the impeller rotates with the shaft.
A conventional ESP assembly includes, from upstream to downstream, a motor, seal section, intake section, and multi-stage centrifugal pump. Production tubing carries the pumped fluid from the centrifugal pump to the well's surface. The assembly components each have a shaft running longitudinally through their centers that are connected and rotated by the motor. In gassy wells, a gas separator or charge pump may also be included in the assembly, typically between the intake and the pump, or in place of the intake. For example, a gas separator may act as the intake of the assembly. In such instances, the gas separator compresses the gaseous fluid and then attempts to separate any unsaturated gas before the fluid passes into the centrifugal pump. Gas separators sometimes include impeller and diffuser stages to increase the pressure of the fluid during compression and separation of gases. Similarly, charge pumps are also sometimes used in tandem with a primary centrifugal pump in gassy wells, and may also employ stages.
During operation, whether in a pump, charge pump or gas separator, pump assembly stages are subject to axial forces in the upward and downward directions, conventionally referred to as “thrust.” Downward force or “downthrust” is a result of a portion of the impeller discharge pressure acting on the top of the impeller. Upward force or “upthrust” is a result of a portion of the impeller discharge pressure acting against the bottom of the impeller. A second upward force is the force produced by the momentum of the fluid making its turn in the impeller passageway. Pump assembly stages are also subject to radial forces that can cause the shaft to become misaligned.
To carry the thrust of the pump, thrust bearings are sometimes employed in pump stages. The thrust bearings include a conventional bushing that is pressed into the wall of the conventional diffuser. A sleeve is keyed to the shaft inward of the conventional bushing. As the sleeve rotates inside the conventional bushing, a thin layer of fluid forms in between the sleeve and conventional bushing of the bearing set to provide fluid film lubrication and carry the downthrust loads. The sleeve may further act as a radial support bearing.
Typically, bushings are tightly pressed into the diffuser bore with an interference fit. The outer diameter of the bushing is larger than the diffuser bore, typically by about 0.001-0.003 inches, and is inserted using a lead in chamfer. Frictional forces between the bushing and the surrounding diffuser attempt to keep the bushing from slipping out of place.
During operation of an ESP assembly, the assembly experiences a significant increase in temperature. In a typical instance, an ESP assembly may be about 75° F. when assembled, but increase to 200-300° F. during downhole operations. Some types of ESP systems reach temperatures as high as 600° F. during downhole operations. Since the various pump components are made of different materials from one another, those materials expand at different rates as the pump increases in temperature. Conventionally, diffusers are made of Ni-resist, an austenitic iron alloy, and bushings are made of a composite material such as tungsten carbide, silicon carbide or titanium carbide. The Ni-resist diffuser has a much higher thermal expansion coefficient than the composite bushing, causing the diffuser to expand faster than the bushing press-fit into the diffuser. The result is that the press-fit on the bushing relieves and the bushing dislodges or rotates. Once the bushing dislodges or rotates, it can cause misalignment or undesirable movement that can degrade the overall performance and/or operational life of the pump.
One approach to combating the dislodgement of ESP bushings due to thermal expansion has been to place a Ni-resist retaining ring above the bushing. Because the retaining ring is the same material as the diffuser, it grows in size with the diffuser, keeping the bushing from sliding upwards as the temperature increases. However, this approach is not possible when the bearing needs to provide thrust protection, as the retaining ring cannot provide any thrust load.
As is apparent from the above, current ESP stages employing press-fit thrust bearings do not adequately prevent bearing dislodgement during temperature increases in downhole wells in which those stages are employed. Therefore, there is a need for an improved press-fit bearing locking system, apparatus and method.